Introduction to dissolved oxygen

Why is oxygen important? For us humans, if we have oxygen, we survive...yay! If not, we don't...boo. So, superficially, this may not seem like a very important parameter to know - you either have oxygen, or you don't. However, for many microorganisms, there are a lot of shades of gray.

For a bacteria or a yeast, different amounts of oxygen produce different results. For instance, starving a yeast cell of oxygen produces ethanol as a metabolite product instead of carbon dioxide. Starving a lake of oxygen not only prevents fishies from living in it, but also promotes the formation of large algae surfaces. Cool, right?

The biggest problem with measuring dissolved oxygen currently is the cost of the equipment available to do it. Typically, dissolved oxygen probes run well into the $400+ range, thus placing them well out of the realm of hobbyists. The cost is not wholly unwarranted - dissolved oxygen meters used a platinum catalyzed reaction with very specific membranes to measure oxygen response. By cutting out the platinum catalyst and the specialized membrane, the cost of a DO meter could drop considerably...enter the optode!

Building a dissolved oxygen probe

How an optode works

In order to reduce cost, we'll be building a dissolved oxygen optode instead of the more common dissolved oxygen electrode.

In an electrode, a small change in a voltage or current is used to detect a change in oxygen concentration. In an optode, a small change in reflected light intensity is used to detect changes in oxygen concentration:

There are advantages and disadvantages to each system.

Properties of commercial dissolved oxygen electrodes:

Very robust - easily waterproofed

Very accurate

Need to recalibrate is rare due to non-reactive nature of the membrane and platinum

Very small amperages are produced - an amplifier circuit must be built at the amp meter position

Very, very expensive - $400 and up

Properties of commercial dissolved oxygen optodes:

Film must be intact for proper sensing - not as robust

Film must be permeable to oxygen, but impermeable to media (i.e. water)

Calibration is difficult - more frequent recalibration necessary due to film degradation

Chemicals in sensing foil respond at visual wavelengths, so background light can interfere with accuracy and precision

Very cheap components needed

No need for an amplifier circuit!

Commercial probes are very, very expensive - $400 and up as well, but based on the design components, could it be made for cheap?

Although the optode has several drawbacks that make it impractical for some uses, there are enough benefits in its simplistic design to make it a potential probe that can handle many situations for $20 or less!

Plan to overcome technical hurdles

Sensor Film Material

Tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) dichloride complex which changes color based on amount of oxygen present

The sensor film material must not only fluoresce when exposed to light, but it must also fluoresce at different intensity levels depending on the amount of oxygen in contact with the film. Thankfully, some really smart chemists have already thought of a material to do this...get ready...its a mouthful! The chemical is formally named Tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II). Since this is such a mouthful, we'll be referring to this chemical in the rest of the document as Ru(dppf) which is chemistry-speak for the above name.

Because ruthenium is a metal missing 2 valence electrons (that's the II in the full name for Ru(dppf)), you can only purchase this chemical in one of many complexes. A complex in chemistry is when ions of an opposite charge are associated to an already charged, but unstable molecule. This creates a bond that stabilizes the unstable molecule making it safe and/or easy to handle. Typically, the complexing agent doesn't change the properties of the molecule its bonded to - it just holds on for the ride.

For our application, choice of a complexing molecule isn't very important, so we foudn the cheapest complex of Ru(dppf) that we could, and ordered some! It happens to be the Ru(dppf) complex (shown to the right).

Light Sealing

In attempting to build a light tight structure, its important to consider where light could potentially leak into your device. It will help in driving performance requirements for the housing. For the optode, light can enter the structure from 3 main points. Remember, for the optode to work, light in the visible spectrum must be blocked!

If the optode film is clear, ambient light from the room in which you are measuring